The present invention relates to a novel cell for polarographic analysis. More particularly, the present invention relates to an automatic cell for polarographic analysis characterized by the very short time required for the analysis in question.
Polarographic analysis was invented about sixty years ago and is defined as a method of analyzing solutions of reducible or oxidizable substances. Today, many chemical elements can be determined by polarographic analysis. The method is also applicable to the analysis of alloys and to many inorganic compounds. It can, in addition, be utilized for the identifications of many organic compounds and for the study of chemical equilibria and rates of reactions in solutions.
The principle of polarography is very simple: The solution to be analyzed is placed in a glass cell containing two electrodes. One electrode consists of a glass capillary tube from which mercury drops slowly (the so-called dropping mercury electrode), the other electrode (the reference electrode) is a pool of mercury or made of some other non-polarizable material.
In recent years, it has become customary to add a third electrode (the counter-electrode) in order to prevent any current flow through the reference electrode.
The cell is connected in series with a galvanometer, which measures the flow of current in an electrical circuit that contains a battery or other source of direct current and a device by means of which the voltage applied to the electrodes can be varied from zero to a few volts. The dropping mercury electrode is usually connected to the negative side of polarizing voltage, and as the voltage is increased by small increments the resulting current is observed on the galvanometer. The current remains very small until the applied voltage reaches a value large enough to cause the substance being determined to be reduced at the dropping mercury electrode. The current then increases rapidly, but it gradually attains a limiting value which remains constant although the voltage is increased further. In qualitative analysis, the voltage required to cause the onset of the rapid increase in current is characteristic of the substance being reduced and serves to identify it. Under appropriate conditions, the final constant limiting current is determined by the rate of diffusion of the reducible substance from the bulk solution to the surface of the mercury drops, and its magnitude is a measure of the concentration of the reducible substance. This renders polarographic analysis quantitative.
About fifty years ago an instrument called polarograph was invented, in which an increasing voltage is applied to the dropping electrode and a curve of current versus voltage (called a polarogram) is recorded.
It is known that electro-analytical methods in general, and sophisticated polarographic methods in particular, such as, e.g. differential pulse polarography, offer several unique advantages over other techniques. They exhibit high sensitivity and high accuracy at moderate to low maintenance costs.
Most instruments utilize the common polarographic cell, which suffers, however from the disadvantage that, prior to the determination, a time-consuming step, the so-called deaeration, is required. This is due to the well known fact that the oxygen dissolved in the solutions to be analyzed interferes with the current measurement. Adequate deaeration--the prior removal of oxygen--usually takes about ten minutes. In a prior communication (Ch. Yarnitzky et al. Anal. Chem. 48 2024, 1976) a description of the operation of a deaeraion device was presented.
Up to now, none of the existing polarographic analyses can be automatically operated, but each sample nevertheless requires much manual handlings as well as deaeration for a relatively long time in the course of its analysis.